The present invention relates to packaging for electronic devices, and more particularly to methods and devices for hermetically sealing packages for electronic chips and circuits that have a plurality of electrical connections.
Electronic devices, and particularly chips, circuits, microcircuits and the like, may operate in environments hostile to reliable device operation. In such environments, it is desirable to place one or more of the devices in a package that is sealed from contaminants such as dust, humidity, air, oxygen and the like. When a package is sealed to protect the device therein from one or more of the contaminants that affect device operation, the package is said to be hermetically sealed.
While various types of packaging materials are known, metal matrix composites are particularly favored because they are engineered materials that may be tailored to have specific properties. For example, compared to ceramic packaging materials, metal matrix composites have about a 30% lower mass density, are stiffer and less brittle, and have a significantly higher thermal conductivity. A ceramic package that has a coefficient of thermal expansion (CTE) matched to the CTE of the substrate of the electronic device thereinside may have a relatively poor thermal conductivity of about 21 W/m.degree.K., while a silicon carbide/aluminum metal matrix composite with a similarly matched CTE would have a thermal conductivity of about 180 W/m.degree.K. Other materials, such as Kovar or molybdenum, may be favored as packaging materials when the amount of packaging material is large and cost is critical, and/or when power dissipation is nominal.
Matching the CTE of the packaging material to the CTE of the portion of the electronic device in contact with the package (e.g., a silicon chip or a substrate connecting several chips) is a widely used technique in the electronics industry. However, as illustrated in the example above, ceramic packages with matched CTEs have a relatively low thermal conductivity and may be unsuitable for electronic devices with high power requirements. Metal matrix composites with matched CTEs are more likely to be able to provide adequate thermal conductivity so that heat generated by the electronic device can be conducted to an exterior heat sink and keep the device within an acceptable operating temperature range.
One of the problems associated with metal matrix composite packages is that they are electrically conductive and the electronic devices inside the package may have a multiplicity (e.g., hundreds) of electrical connections that must be made through the packaging material. If the packaging material is conductive or semi-conductive, each electrical connection through the package must be insulated from the packaging material to prevent electrical shorts to the package. This problem is exacerbated where the package is to be hermetically sealed because each passageway through the package for an electrical connection (i.e., a feedthrough) must also be hermetically sealed.
With reference to FIG. 1, one known technique for insulating the electrical connections through a package 10 for an electronic device 12 is to provide separate feedthroughs 14 for each electrical connection 16. (See, for example, U.S. Pat. No. 4,506,108 issued Mar. 19, 1985 to Kersch, et al.) The feedthroughs 14 may be electrically insulated from the package 10 by insulative material 18, such as glass beads, which surround the feedthrough, supporting and insulating it, as it penetrates the package. As is apparent, however, the number of electrical connections that can be made is limited because the distance between feedthroughs must be sufficient for the insulative material 18 surrounding adjacent feedthroughs 14. For example, the minimum center to center spacing achievable using this technique with glass beads is only about 0.050 inches, yielding a density of about 20 electrical connections per inch. When hundreds of electrical connections are to be made, the size of the package must be increased and such large packages are not favored in many applications, particularly in light of the increasing demand for miniature integrated circuit packages. In addition, the insulative material such as glass may crack during lead formation and/or shock experienced by the package.
Packages with multiple connections per feedthrough (instead of just one connection per feedthrough) are also known. (See, for example, U.S. Pat. No. 4,412,093 issued Oct. 25, 1983 to Wildeboer). With reference to FIG. 2, prior art feedthroughs include plural electrical connectors 20 that extend all the way through a single glass insulator 22. The feedthrough is thereafter inserted into an opening 24 in the frame 26 for the package and a glass-to-metal seal formed to seal the end of the package. An electronic device may be thereafter placed in the frame 26, electrical connections made, and a lid installed to hermetically seal the device.
However, the connectors 20 must have sufficient structure to be self-supporting so they rigidly extend from both sides of the glass insulator 22 and large enough to be joined to an exterior component (e.g., by solder, weld, bonding, etc.), requirements that are likely to be met only by the board-like electrical connectors 20 illustrated in FIG. 2 or the large diameter wires of FIG. 1. Additionally, the connections must be spaced sufficiently to permit the glass to flow between adjacent connectors to form a self-supporting structure. While electrical connection density may be increased somewhat, the size restrictions imposed by the rigidity, connection, and flow requirements still reduce the number of connectors that may be carried by a single feedthrough below that needed for some applications. The device is also susceptible to some of the other problems associated with the single connector feedthroughs, such as cracking of the glass insulator 22.
Accordingly, it is an object of the present invention to provide a novel feedthrough having multiple electrical connections that obviates these and other problems of the prior art.
It is another object of the present invention to provide a novel method for packaging an electronic device that obviates the problems of the prior art.
It is a further object of the present invention to provide a novel feedthrough having multiple electrical connections through a multilayer cofired ceramic structure.
It is still a further object of the present invention to provide a novel packaging and device having multiple electrical connections that extend from inside a package to outside the package and that are small enough to significantly increase the number of such connections that can be accommodated without increasing package size.
It is yet a further object of the present invention to provide a novel method of making a feedthrough having multiple electrical connections by connecting vias to electrically conductive metal paste paths that are between two or more ceramic green sheets.
It is still another object of the present invention to provide a novel method of hermetically sealing a package for an electronic device in which a feedthrough having multiple electrical connections is formed by connecting vias to electrically conductive paths that are between two or more ceramic layers and is inserted into an opening in the package frame and hermetically sealed therein.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.